Hello! Aside from the typical website platforms, I would recommend using Twitter's hashtags and user handles. Try following and using the ones that your potential prospect or otherwise ideal partners would be following and start sharing about your work, your progress, and outreach for a potential partner. If possible try to be local when doing so.
But obviously, some to use include Startups.co, Basecamp, Inc 500, yesPHX, BetaBulls, MPV, Lean Startup, Lean Methodologies, TechCrunch, etc.

It comes down to the SKA’s and the PIP’s. My HR professor at UMass, Dr. Fernsten, instilled this in us from day one. You should hire people based on PIP’s, not SKA’s, if you feel they can do the job with proper training.

What are PIP’s? They are Personality, Interests and Preferences. These are the natural fiber of someone, and are often less malleable than SKA’s – Skills, Knowledge and Abilities. Hiring people to fit your organization, who love your organization, and have the personality and interest to join that organization is so important; if they have the right character, you can often train them to do ANYTHING.

I recall a position I took back in 2005 working for a consulting company. The initial call was for someone to fix and maintain their computers and to update their website. At the time, I had little or no experience with websites, but I had good business sense, was getting my MBA, and wanted to work for this company. They brought me in for my PIP’s, not by SKA’s. I still work for them, and one of the comments from them was that I really didn’t know a lot about websites when I started with them, but I demonstrated interest, and they found me to be trustworthy and had the right personality for the work.

If you're looking to find a business partner I would recommend even before starting your search to think about and analyze why you are interested in bringing on a partner. Are you looking to find a complimentary skillset? or are you bringing someone on to take work off your hands?

Understanding what you are looking for and who you hope to meet is the first step in a successful partnership arrangement.

Once you've decided on what you are looking for it's time to be aggressive. In a recent search for a technical co-founder, my partner and I effectively created a sales pipeline filled with potential candidates. We did a majority of our work looking online finding potential partners on Angel List, Built in and Co-Founders Lab and complimented that with attending meet-ups and other in-person events. We developed a list of about 40 potential technical partners and reached out to them 1 by 1 tracking our outreach. A majority of those we reached out to turned us down, that was the bad news. The good news was because we reached out to 40! people even though a majority turned us down we still had a large enough pool that we could find the person we thought would work best with our team.

Side Note- Once you have reached out to potential partners who are interested in working on your project, your job isn't done. You need to get them to understand your business and get them to buy in before you can move forward.

There are a couple of options. These options are visiting meetups, startup events, using your network and finding them online. I will give you a quick list of sites that you can use to find your business partner.

Founder2be - Founder2be is a website that allows both startups and individuals to find one another. Hence, it allows their users to find co-founders. Users can specify the country and position they are looking for. Then, they can scroll through individuals that match that criteria.

Founders Nation - Very similar to Founder2be, this site allows users to look for people that are interested in joining startups. Each person has a profile that states their skills, what they are looking for/what they want to do, their stage, contact details, location and much more.

Work in Startups - A job board listing for startups and startup jobs. There is a whole section for startups looking for co-founders. The main focus of the site is for people looking to join businesses. Hence, it would be a good idea to advertise the position on this site.

Angel List - Similar to Work In Startups, Angel list allows their users to list their company and then advertise hiring positions. Furthermore, Angel List can be useful if you are ever interested in looking for investors. The main focus of this site is to get people that have the same passion for startups together.
Like I said earlier, don’t forget to visit meetups and startup events as well. The face to face contact can be extremely useful for finding your business partner!